A wavelength filter for extracting a specific wavelength can be realized for example using wavelength selectivity of diffraction gratings with various forms. FIGS. 11(a).about.11(c) show a conventional configuration in which a diffraction grating is formed and disposed to be adjacent to a waveguide and also its effective width (referred as coupling width hereinafter) to be constant in the light propagation direction z. FIG. 11(a), FIG. 11(b) and FIG. 11(c) respectively show its plan view, front view, and right side view. Aplane diffraction grating 12 as shown in FIG. 11(a) is formed on a substrate 10, and then a waveguide 14 is formed on it. It is widely known that a cladding layer is sometimes formed on the side of the waveguide 14. The width of the diffraction grating 12 is constant for the distance L of the light propagation direction z.
The diffraction grating 12 can be easily formed using conventional semiconductor crystal growth technology. For instance, after forming a film with a predetermined thickness on the substrate 10, a periodic corrugation should be made using etching. Then the diffraction grating 12 is formed by crystal-growing material having different refractivity on the corrugation. There are several configurations for realizing the waveguide 14 itself. The waveguide 14 is logically illustrated in FIGS. 11(a).about.11(c) . The diffraction grating 12 is sometimes formed on the waveguide 14.
FIG. 12 shows wavelength selective characteristics of a wavelength filter employing a diffraction grating with a constant coupling width as shown in FIGS. 11(a).about.11(c). The abscissa axis shows wavelengths, and the ordinate axis shows output intensity. A central wavelength is 1.55 .mu.m. It is simulated for the structure that InGaAsP waveguides #1 and #2 (their band gap wavelength .lambda.g =1.24 .mu.m) respectively having a rib with a width of 4 .mu.m and a height of 0.2 .mu.m are formed 0.8 .mu.m apart from each other and a diffraction grating is disposed between the waveguides #1 and #2 as shown in FIG. 13. The length L of the diffraction grating is preset to 3 mm. The InGaAsP waveguides #1, #2 and the diffraction grating are imbedded in InP. FIG. 13 shows the structure in the surface orthogonal to the light propagation direction. FIG. 12 shows an output optical spectrum when backward coupling between the waveguides #1 and #2 is employed. It is clear from FIG. 12 that, in a conventional configuration in which the coupling width of the diffraction grating is constant in the light propagation direction, the wavelength selectivity is insufficient and the side mode is not effectively suppressed.
FIGS. 14(a) 14(c) show a conventional configuration in which a coupling width of a diffraction grating linearly widens and narrows in the light propagation direction z. Similarly to FIGS. 11(a).about.11(c), FIG. 14(a), FIG. 14(b) and FIG. 14(c) respectively show its plan view, front view, and right side view. In this conventional configuration, also, a plane diffraction grating 22 shown in FIG. 14(a) is formed on a substrate 20, and then a waveguide 24 is formed on it. It is widely known that a cladding layer is sometimes formed on the side of the waveguide 24.
The coupling way of the diffraction grating 22 with the waveguide 24, namely the coupling width of the diffraction grating 22 varies in the light propagation direction. That insufficient.
Outputs of the conventional wavelength filters still contain many unnecessary components of other wavelength bands besides a selected wavelength, therefore the improvement of the wavelength filter has been expected.